The present invention is in the field of lasers, and more particularly, in the field of lasers in which the lasing medium is an optical fiber which is pumped with a pump optical signal to generate an output signal that is detected by a detector.
Optical fibers are being used for an increasing number of applications. One such application is an optical fiber rotation sensor comprising a loop of optical fiber into which two light signals are introduced and caused to counterpropagate around the optical loop. Such rotation sensors are described, for example, in U.S. Pat. No. 4,410,275; U.S. Pat. No. 4,456,377; U.S. Pat. No. 4,487,330; U.S. Pat. No. 4,634,282; and U.S. Pat. No. 4,637,722. These patents are hereby incorporated by reference herein. For such rotation sensors and for other optical fiber applications, it is desirable to have a stable well-controlled light source.
Prior art lasers typically concentrate the optical energy output from a laser in a very narrow band of optical wavelengths and have a relatively long temporal coherence length. In addition, some lasers are tunable over a range of wavelengths. For example, L. Reekie, et al., "Diode Laser-Pumped Operation of an Er.sup.3+ -doped single-mode fibre laser,", Electronics Letters, Sep. 24, 1987, Vol. 23, No. 20, pp. 1076-1077, which reports on extending the tuning range of an Erbium-doped fiber by varying the fiber length. In C. A. Millar, et al., "Low-Threshold cw Operation of an Erbium-Doped Fibre Laser Pumped at 807 nm Wavelength," Electronics Letters, Jul. 30, 1987, Vol. 23, No. 16, pp. 865-866, a low-threshold Erbium-doped optical fiber laser is described that has a linewidth of about 1 nm FWHM (Full Width at Half Maximum). In Laurence Reekie, et al., "Tunable Single-Mode Fiber Lasers," Journal of Lightwave Technology, Vol. LT-4, No. 7, July 1985, pp. 956-959, the tuning characteristics of a Nd.sup.3+ -doped single-mode fiber laser and the tuning characteristics of an Er.sup.3+ -doped single-mode fiber laser are described. In R. J. Mears, et al., "Neodymium-Doped Silica Single-Mode Fibre Lasers," Electronics Letters, Vol. 21, No. 17, Aug. 15, 1985, pp. 739-740, a dye-laser pumped fiber ring-cavity laser having a FWHM linewidth of 2 nm at a wavelength of 1078 nm is described. In David N. Payne, "Special Fibers and their uses," OFC/IOOC '87, Reno, Nev., Jan. 21, 1987, Invited Paper WI1, the use of rare-earth and transition-metal-doped single-mode fibers in very long (300 meters) lasers that are tunable over a range of 80 nm is discussed. In C. A. Millar, et al., "Tunable Fiber Laser," Technical Digest of Conferences on Lasers and Electrooptics, Paper WD2, Apr. 29, 1987, a Nd.sup.3+ -doped Ge/P.sub.2 O.sub.5 silica single-mode fiber is tuned by incorporating a broadband dielectric mirror in the laser cavity through which the fiber is pumped using an Ar-ion laser at a wavelength corresponding to a strong absorption band of Nd. The output consists of a dominant line of approximately 1 nm width (FWHM) centered around 1067.5 nm, corresponding to the maximum reflectivity of the fiber mirror. In R. J. Mears, et al., "High Power Tunable Erbium-Doped Fiber Laser Operating at 1550 nm," Technical Digest of Conferences on Lasers and Electrooptics, Paper WD3, Apr. 29, 1987, a CW Q-switched fiber Erbium-doped fiber laser is described which is tunable about the wavelength of 1550 nm using a holographic grating. An acousto-optic modulator is inserted into the laser cavity to Q-switch the laser. In R. J. Mears, et al., "Low-Threshold Tunable CW and Q-Switched Fibre Laser Operating at 1.55 microns," Electronics Letters, Volume 22, Number 3, pp. 159-160, 1986, an intracavity objective is used to collimate light through an acousto-optic modulator onto a holographic grating. The fiber laser is tuned by varying the angle of the diffraction grating. Q-switching is effected by operating the acousto-optic modulator. In F. V. Kowalski, et al., "Optical Pulse Generation With a Frequency Shifted Feedback Laser," Applied Physics Letters, Volume 53, Number 9, pp. 734-736, Aug. 29, 1988, optical pulses are also generated using an acousto-optic modulator that shifts the frequency of the light inside the laser cavity of a dye laser. Wavelength tunable operation of an Erbium-doped fiber laser has also been demonstrated using a tunable ring resonator configuration as described in P. L. Scrivener, et al., "Narrow Linewidth Tunable Operation of Er.sup.3+ -Doped Single-Mode Fibre Laser," Electronics Letters, Volume 25, Number 8, pp. 549-550, Apr. 13, 1989. All of the above articles are hereby incorporated by reference herein.
For some applications, such as certain optical fiber rotation sensors, a high power broadband optical energy source having a short temporal coherence length and no longitudinal mode structure at longer wavelengths is desirable. It has been demonstrated that using a broadband optical energy source in an optical fiber rotation sensor, for example, reduces phase errors caused by the Kerr effect. A broadband optical signal can also be advantageously used to reduce phase errors in the combined optical signal from the loop of the rotation sensor caused by coherent backscattering (i.e., Rayleigh backscattering) and by polarization cross-coupling in the loop. See, for example, U.S. Pat. No. 4,773,759; U.S. patent application Ser. No. 488,732, filed on Apr. 26, 1983; (now abandoned) and U.S. patent application Ser. No. 909,741, filed on Sep. 19, 1986, now U.S. Pat. No. 4,881,817 issued November 1989; all of which are assigned to the assignee of the present application. These patents and patent applications are hereby incorporated by reference herein. A theoretical analysis regarding the broadband source requirement for fiber gyroscopes can be found in W. K. Burns, et al., "Fiber-Optic Gyroscopes with Broad-Band Sources," Journal of Lightwave Technology, Volume LT-1, Number 1, pp. 98-105, March 1983. This article is hereby incorporated by reference herein. Optical fiber rotation sensors also require highly stable center wavelengths with little thermal drift. A rotation sensor source must also have the ability to couple high power into the rotation sensor without creating large noise components (high signal/noise ratio). Finally, an ideal rotation sensor source preferably operates in higher wavelength region of the output spectrum of the source in order to reduce any radiation effects.
Such broadband optical sources include, for example, superluminescent light emitting diodes, and the like. An exemplary superluminescent diode has a relatively broad optical linewidth (e.g. approximately 15 nm) at the optical wavelengths in the range of 800 to 850 nm, for example. However, for a given power input, exemplary superluminescent diodes may not provide an adequate amount of optical energy when compared to a laser, for example. More importantly, superluminescent diodes cannot be easily coupled to certain optical devices such as gyroscopes as the light emitted by superluminescent diodes is highly divergent. On the other hand, resonant cavity lasers typically provide adequate amounts of power but have a relatively narrow linewidth (e.g., less than 5 nm). Furthermore, lasers can be easily coupled to fiber optic gyroscopes. It is desirable to obtain the relatively high output and the coupling capacity of a laser while obtaining the relatively wide linewidths heretofore unobtainable with a resonant cavity laser. Furthermore, it is known that the temperature stability of the emission wavelength of a typical superluminescent diode is not satisfactory for numerous applications. It is also desirable that the emission wavelength be stable over a wide temperature range.
More recently, U.S. Pat. No. 4,637,025 to Snitzer, et al., describes a superradiant light source that includes an optical fiber having a core doped with a selected active laser material such as Neodymium.
U.S. patent application Ser. No. 281,088, filed on Dec. 7, 1988 and now U.S. Pat. No. 4,938,556 issued on Jul. 3, 1990, discloses a superfluorescent broadband fiber laser source comprising a fiber doped with laser material coupling to a multiplexing coupler. This application is assigned to the assignee of the present application. Such a superfluorescent source has good output power and easily couples to an optical fiber rotation sensor. It does not have longitudinal cavity modes and shows good thermal stability. Its spectrum is much broader than a resonant laser source. The above patent and patent application are hereby incorporated by reference herein.
Superfluorescent sources, however, require relatively high pump power, tend to lase as the result of reflections from lenses or other fiber ends causing narrowed spectrum and coherence. Furthermore, a special narrowing of the spectrum occurs as the result of selective amplification of peak frequencies at high powers in some materials (such as Erbium). Finally, the design of a superfluorescent source requires careful consideration of the variation of the output spectrum as a function of fiber length and output power.
One skilled in the art will recognize that the use of superfluorescence in an optical fiber light source will not provide an optical signal output intensity as high as can be obtained by a resonant cavity laser in which the oscillating light induces further emissions of light at the output wavelength. However, it has been previously understood that the use of a resonant cavity laser will produce an optical output signal having an undesirably narrow spectrum, thereby making resonant cavity lasers disadvantageous for use as broadband light sources.
U.S. patent application Ser. No. 176,739, filed on Apr. 1, 1988 (now abandoned), describes a broadband light source which uses an optical fiber doped with a lasing material such as Neodymium. This application is assigned to the assignee of the present application and is hereby incorporated by reference herein. The optical fiber is pumped with a pump optical signal having a pump wavelength selected to cause spontaneous emission of an optical signal at a second wavelength different from the pump wavelength. The wavelength of the pump optical signal is selected to be outside the pump variable tuning range of the Neodymium-doped optical fiber (i.e., the range of pump wavelengths which stimulate emitted wavelengths having an average wavelength with a generally one-to-one correspondence to the pump wavelength). Pumping with a pump signal outside the pump variable tuning ranges causes the emitted light to have a broad spectral envelope of longitudinal modes having emission wavelengths corresponding to substantially all the pump variable tuning range.
In recent years, Erbium-doped fiber lasers have received increasing attention as possible laser source and for amplification purposes in the low loss fiber communication window at 1500 nm. It is possible to obtain a high gain when the Erbium dopant is properly doped into the fiber, typically a silica fiber. The light emitted by Erbium-doped fibers easily couples into other fibers with similar mode sizes. An Erbium-doped fiber is also thermally relatively stable. Additionally, Erbium-doped fibers emit higher wavelength light than Neodymium-doped fibers, which makes them less sensitive to radiation induced loss mechanisms. Unfortunately, Erbium in silica is a mostly homogeneously broadened transition so that operation as a resonant laser produces a narrow linewidth measured at around 0.23 nm to 2 nm for either line of the spectrum. When operated as a high power, nonresonant, amplified spontaneous emission source, the Erbium fluorescence line is produced at longer wavelengths and has a short temporal coherence length and no longitudinal mode structure. This line seemingly meets the requirements imposed by certain optical fiber applications such as rotation sensors. The fluorescence spectrum of Erbium exhibits two narrow peaks at 1530 nm and between 1550 nm and 1560 nm. Near the 1530 nm peak, the linewidth is in the range of 4-5 nm. The narrowing of the fluorescence spectrum with increased input pump power indicates a mostly homogeneously broadened transition. For shorter lengths of the fiber, the 1530 nm peak is dominant. However, for longer lengths of the fiber, the 1550-1560 nm peak becomes dominant and has a wider linewidth on the order of 10-15 nm. Although such a source has a much broader spectrum than the resonant structure, it has limited power capabilities. These narrow peaks are inadequate in certain optical applications such as rotation sensors that require broader bandwidths in the range over 20 nm.
Recently, the transition of Erbium-doped fibers near 1550 and 1530 nm has been tuned by incorporating a rotatable grating or a grating with rotatable mirror to provide angle tuning capabilities. Electronic tuning of such a source is however preferable because it facilitates an all electronic laser wavelength control system with increased flexibility. As mentioned above, wavelength tunable operation of an Erbium-doped fiber laser has also been demonstrated using a tunable ring resonator configuration as described in the aforementioned Scrivener reference.
Electronic tuning has been implemented by means of an acousto-optic modulator. Electronic wavelength tuning was first demonstrated in an inhomogeneously broadened dye laser. In an article by D. J. Taylor, et al., "Electronic Tuning of a Dye Laser Using the Acousto-Optic Filter," Applied Physics Letters, Volume 19, Number 8, Oct. 15, 1971, there is described a dye laser which is electronically tunable over 780 Angstrom by inserting an acousto-optic filter into the dye laser cavity. This article is hereby incorporated by reference herein. When feeding back the frequency shifted deflected beam of the intracavity Bragg cell, the usual cavity longitudinal modes no longer exist but are replaced by frequency chirped modes. The acousto-optic filter causes the frequency of the light deflected by the Bragg cell to shift by twice the acoustic frequency in a round trip and so the usual laser resonance condition is not satisfied. According to the aforementioned Taylor reference, two modes of operation, however, can be distinguished, depending on various parameter values. In the first case, the dye saturates and the output is laserlike, whereas in the second case, there is amplified spontaneous emission (also referred to as superfluorescence). A theoretical analysis of the tunability of a dye laser tuned by acousto-optic filter can be found in W. Streifer, et al., "Analysis of a Dye Laser Tuned by Acousto-Optic Filter,", Applied Physics Letters, Number 17, pp. 335-337, Oct. 15, 1970, in P. Saltz, et al., "Transient Analysis of an Electronically Tunable Dye Laser--Part I: Simulation Study," IEEE Journal of Quantum Electronics, Volume QE-8, Number 12, pp. 893-899, December 1972, and in W. Streifer, et al., "Transient Analysis of an Electronically Tunable Dye Laser--Part II: Analytic Study," IEEE Journal of Quantum Electronics, Vol. QE-9, Number 6, pp. 563-569, June 1973. A thorough review of electronically tunable lasers can also be found in D. J. Taylor, "Electronically Tunable Lasers," Dissertation submitted to the Department of Electrical Engineering of Stanford University, December 1972. These articles are hereby incorporated by reference herein.
There is therefore the need for a high power broadband light source with short temporal coherence lengths and no longitudinal mode structure, having one or more stable high center wavelengths with little thermal drift which preferably uses an optical fiber structure. The present invention advantageously uses an intracavity frequency-modulated acousto-optic modulator to produce a broadband light source from a narrow-lined laser medium.